1. Field of the Invention
The present invention relates to a carrier which is used for developing a latent electrostatic image formed on a photoconductor to form a visible image; a developer containing the carrier and a toner; a container housing the developer; and an image forming process, an image forming apparatus, and process cartridge using the developer.
2. Description of the Related Art
In the prior art, in electrophotography apparatuses and electrostatic recording apparatuses, an electrical or magnetic latent image is rendered visible by a toner. For example, in the electrophotography process, an electrostatically charged image (latent image) is formed on a photoconductor, and this latent image is then developed using a toner to form a toner image. This toner image is usually transferred to a transfer material such as paper, and subsequently fixed by a process such as heating.
In general, the toner used for the electrostatically charged image development comprises coloring particles containing a colorant, charge control agent and other additives in a binder resin, and may be manufactured, broadly speaking, by a pulverization process or a suspension polymerization process.
In the pulverization process, toner is manufactured by melt-mixing the colorant, charge control agent and offset inhibitor, uniformly dispersing them in a thermoplastic resin, and then pulverizing and classifying the composition obtained. By the pulverization process, a toner having excellent characteristics can be manufactured, but selection of toner materials is limited.
For example, the composition obtained by melt mixing must be a material which can be pulverized and classified by an economically viable apparatus. Due to this requirement, the composition which is melt-mixed must be sufficiently brittle to be pulverized and classified.
Thus, when the composition is actually pulverized to particles, a particle size distribution with a wide range is easily formed, and if it is attempted to obtain a copy image with good resolution and gradation, fines with a particle size of for example 5 μm or less and coarse powder of 20 μm or more must be removed, so the yield becomes very low. Additives such as a coloring agent and charge control agent cannot be uniformly dispersed in a thermoplastic resin according to the pulverization process. Uneven dispersion of the components has an adverse effect on toner fluidity, development and durability, and image quality.
Recently, to overcome the problem in these pulverization processes, it was proposed to manufacture the toner by a polymerization process such as suspension polymerization process, and this process is now being used. Such toner particles for electrostatic development are prepared, for example, by suspension polymerization. However, the toner particles obtained by the suspension polymerization process are spherical, and they are not easy to clean off. In development and transfer of an image with a low image occupancy, the amount of a residual toner after transfer is small and the cleaning failure does not become a problem. However, in development and transfer of an image with a high image occupancy or in the case that a toner constituting an image is not transferred due to paper feed failure and remains on the photoconductor, the cleaning failure leads to toner deposition on the background of images. In addition, such a residual toner may be deposited on a charger roller for contact-charging the photoconductor and other members, thus reducing the inherent charging ability of them.
To solve these problems, Japanese Patent No. 2537503 discloses a process of preparing fine resin particles by emulsion polymerization and associating the fine resin particles with one another to thereby yield toner particles having irregular shapes.
However, even if the toner particles obtained by emulsion-polymerization are subjected to a water rinsing process, as surfactant remains not only on the surface but also in the interior of the particles, the environmental stability of the toner charge is adversely impacted, the charge distribution is broadened, and the obtained image is soiled. Moreover, the remaining surfactant also contaminates the photoconductor, charge roller and developing roller, so that they are not able to manifest their original charging capability.
Toner particles for use in an image-fixing process, in which an image is fixed by contact heating using a heating member such as heating roller, must have satisfactory releasing properties with respect to the heating member (hereinafter may be referred to as “anti-offset performance”). The anti-offset performance can be improved by arranging a releasing agent on the surface of toner particles. Japanese Patent Application Laid-Open (JP-A) No. 2000-292973 and No. 2000-292978 each propose a process for imparting the anti-offset performance to toner particles by arranging fine resin particles mainly in a surface layer of toner particles in addition to compounding them within the toner particles. However, these techniques invite an elevated lowest image-fixing temperature, thus deteriorating properties in low-temperature image-fixing systems for energy saving.
The processes, in which toner particles having irregular forms are prepared by associating resin particles prepared by emulsion polymerization, have the following disadvantages. When particles of a release agent are associated to improve the anti-offset performance of the toner, the release agent particles are included inside the toner particles, and thereby the anti-offset performance of the toner cannot be improved satisfactorily. In addition, since resin particles, release agent particles and coloring agent particles are randomly mixed and fused to form toner particles, the composition of the toner particles varies (i.e., contents of the toner constituents in the toner particles vary) and in addition the molecular weight of the binder resin in the toner particles varies. As a result, the individual toner particles have different surface properties, and therefore the toner cannot stably produce good images over a long period of time. In an image forming system which requires the toner to have a low temperature fixability, the toner prepared by arranging fine resin particles on the surface thereof invites poor image-fixing due to the toner particles having the resin particles unevenly present on their surface, and therefore the toner cannot be used therefor because of having a narrow fixing temperature range.
Developing systems in the electrophotography are roughly divided into one-component developing systems using a toner alone as a main component and two-component developing systems using a mixture of a toner and a carrier such as glass beads and magnetic particles with or without resin coating.
The two-component developing systems use a carrier with a wider contact charging area with respect to the toner, have stable charging properties and are advantageous for yielding high-quality images over a long period of time as compared with the one-component developing systems. They can highly feed the toner to a developing region and are often used in high-speed machines.
The two-component developing systems are also widely employed in digital electrophotographic systems in which a latent electrostatic image is formed on a photoconductor typically using laser beams and is developed to form a visible image.
Such latent images must have smaller minimum unit (one dot) with higher density to produce images with higher resolution and higher highlight reproducibility or to produce color images. A strong demand has therefore been made to provide developing systems that can reproduce these latent images (dots) in exact accordance. To satisfy the demand, various proposals have been made on process conditions and developers (toners and carriers). Relating to the process conditions, a smaller developing gap, a thinner photoconductor, and a smaller diameter of write beams are effective. However, these techniques lead to higher cost and have still insufficient reliability.
Relating to developers, the use of a toner having a small particle diameter significantly improves the reproducibility of dots. However, such a developer containing a toner with a small particle diameter may invite toner deposition on the background of images and insufficient image density. A full-color toner with a small particle diameter uses a resin with low softening point to produce sufficient colors but invites a larger amount of spent toner on the carrier, thus deteriorating the developer and often inviting scattering of toner particles and toner deposition on the background of images.
Various proposals have been made on the use of carriers with a small particle diameter. The carriers with a small particle diameter have the following advantages.
(1) These carriers have a large surface area and can thereby impart sufficient charges to individual toner particles by friction, thus avoiding toner particles with a low charge or with an opposite charge. They can therefore satisfactorily reproduce dots (images) with less toner deposition on the background of images and with less dust and bleeding of the toner around the dots.
(2) These carriers have a large surface area and can prevent toner deposition on the background of images. Therefore, the toner particles can have a decreased average charge to produce sufficient image density. Accordingly, the carriers with a small particle diameter can supplement the disadvantages of and enhance the advantages of the toners with a small particle diameter.
(3) The carriers with a small particle diameter can form fine magnetic brush with good flowability, thus avoiding traces or marks of the magnetic brush on images.
Along with the use of carriers with a small particle diameter having the above advantages, various materials of core particles for carrier particles have been proposed to reduce loads on the environment. More specifically, a Cu—Zn ferrite has been often used for the material for core particles, but it is now less used, due to its constitutional copper and zinc elements. Mn ferrites are now often used instead of the Cu—Zn ferrite. The Mn ferrites often include Mg among various additives to improve their properties. For example, Japanese Patent No. 3243376 discloses a technique of adding Mg and Sr to a Mn ferrite to reduce variation in magnetization among carrier particles. Various improvements thus have been made on the Mn ferrites with an increasing use thereof to thereby improve their quality. However, these Mn ferrites have a low resistance and invite image failure such as irregular image density in a halftone image when their magnetic properties are set within regular usable regions.
Conventional carriers with a small particle diameter often invite deposition or scattering of carrier particles, which causes damage on the photoconductor or image-fixing roller, and are thereby difficult to use in practice.
To solve these problems, Japanese Patent No. 3243376 proposes a specific carrier having a volume-average particle diameter of 25 to 45 μm, containing particles with a particle diameter of 22 μm or less in an amount of 1% or less and having magnetization of 67 to 88 emu/g in a magnetic field of 1 kilooersted, in which scattered particles have a magnetization 10 emu/g lower than that of inherent particles. This technique can reduce deposition of carrier particles but significantly invites “rough image”, a spotted image density irregularity in a test on an analogue halftone image using a developing system in which a direct-current voltage is superimposed with an alternating-current voltage as developing bias. The analogue halftone image in a digital machine used in this test is in the similitude of a digital image with high precision of 1200 dpi or more and the test is a forced test for next-generation digital images with higher precision. In contrast, the rough image is trivial in low-precision digital image formation of around 400 dpi.
JP-A No. 2002-296846 proposes a technique for uniformizing a halftone image by reducing the particle diameter of carrier. In this technique, the irregularity in halftone images is considered to be caused by varying particle diameters. In contrast, the concern in the present invention is irregularity in halftone images caused by electrical factors. To verify the difference between them, the present inventors have made a test on a copier CF-70 (available from Konica Minolta Business Technologies, Inc.) used as a test machine in JP-A No. 2002-296846 and have found that the CF-70 is a full-color copier with a resolution of 400 dpi, and the irregularity in halftone image which is a problem to be solved by the present invention is not observed therein.
Generally, digital images can be reproduced in exact accordance with inputted images more satisfactorily with an increasing resolution of images. This is also true in electrophotography, and investigations on images with higher resolution of 1200 dpi or higher have revealed that smooth images can be produced in highlight or halftone densities at such a high resolution.
However, a higher resolution alone is insufficient to yield a higher image quality, and individual dots constituting the image must have high dot uniformity. The term “dot uniformity” used herein means that each dot bears a toner in an amount with less variation from dot to dot. In an image with a higher resolution, each dot bears a decreasing amount of a toner with a decreasing diameter of dot as compared with an image with a lower resolution. Target high-quality images with smooth entire appearance can be obtained by uniformizing the amount of toner in each dot. In contrast, if each dot bears a toner in an amount largely varying from dot to dot, the difference between the amounts of the toner leads to images with uneven densities. In this connection, images with a lower resolution are not so affected by irregular densities caused by decreased dot uniformity, since each dot in these images bears a larger amount of the toner. To produce high-quality images with higher resolution, investigations have been made to improve the dot uniformity of individual dots.
The “roughness (rough image)” as evaluated in the present invention is a phenomenon in which a rough irregularity in density occurs in images of highlight to halftone densities and which is caused by a decreased dot uniformity. The rough image tends to occur in images with high resolution. The analogue halftone image as tested in the present invention corresponds to an output image of the highest resolution. If the roughness can be improved in the analogue halftone image, high-quality images with high resolution can be produced.
The copier CF-70 is a machine for producing images with a resolution of 400 dpi (one dot: about 60 μm) and does not invite the rough image. More specifically, the irregularity in halftone images observed in JP-A No. 2002-296846 is caused by the difference in particle diameters, and the copier used in this technique cannot detect the rough image, i.e., the irregularity in halftone images caused by electrical factors. Accordingly, the technique is not a solution to the problems in the present invention.
Ferrite carriers such as Ni—Zn ferrite, Mn—Zn ferrite or Cu—Zn ferrite have a dielectric breakdown voltage of 1000 V or more, can avoid leakage of the potential of latent electrostatic image on a photoconductor to the carrier during development, and do not invite brush strokes. However, these ferrite carriers have an excessively high density. To avoid this disadvantage, JP-A No. 07-225497 discloses the use of a Li—Fe ferrite containing 17.0 to 29.0% by mole of lithium oxide to Fe2O3 and describes that such a Li ferrite has a saturation magnetization of about 43 to 70 emu/g (Am2/kg). In the examples and comparative examples in the publication, the maximum saturation magnetization is 62 Am2/kg under the application of a magnetic field of 3000 oersteds. It is highly possible that the ferrite disclosed in JP-A No. 07-225497 will have a saturated magnetization of less than 70 Am2/kg when determined at 1000 oersteds. Accordingly, this ferrite is a low-magnetized ferrite and is distinguished from a high-magnetized ferrite used in the present invention.
JP-A No. 11-202559 described that a Li—Fe ferrite often shows varying properties, since the Li component is susceptible to humidity and temperature, and discloses a ferrite containing 5 to 35% by mole of MoO, 10 to 45% by mole of MgO and 45 to 55% by mole of Fe2O3 to avoid this problem. However, this ferrite is a low magnetization ferrite and is distinguished from the ferrite used in the present invention.
JP-A No. 06-35230 and No. 06-51563 disclose carriers mainly comprising a ferrite and having specific average particle diameter, bulk density and intensity of magnetization. However, these carriers are mainly intended to prevent adhesion or deposition of carrier particles to a latent electrostatic image bearing member such as photoconductor and do not still have a sufficient resistance.